This invention relates generally to hydraulic test equipment. More particularly, it relates to a method of using hydraulic test equipment to measure hydraulic leaks in hydraulic valves and assemblies.
Hydraulic valves and other hydraulic equipment typically include several closely fitting individual mechanical parts that regulate the flow of hydraulic fluid. By opening and closing internal passages formed therebetween, these parts regulate hydraulic fluid flow in a manner that provides the specific functions the operator desires.
Since these components are mechanical, however, and since fluid under pressure is applied to them, they always exhibit a certain amount of leakage through the gaps between the internal components. Eliminating all leakage in hydraulic components would require extremely tight tolerances between the mechanical parts between which hydraulic fluid would otherwise leak. These tolerances would make virtually every hydraulic component extremely expensive.
For this reason, the design tolerances for hydraulic components are increased with the understanding that there will be some residual. Even so, excessive leakage, i.e. that beyond the design limits, is not tolerated.
For this reason, hydraulic components are typically designed to have a specified maximum hydraulic fluid leakage rate to be measured under predetermined conditions. The leakage flow rate is a tolerance like any of the dimensional tolerances of the mechanical components making up the hydraulic device.
The leakage rate itself is a function of the mechanical interaction of all the components making up the hydraulic device. It is the spaces between each of the mechanical components that cause leakage. For this reason, the leakage rate can only be measured and the hydraulic components can only be determined to have passed or failed their leakage rate specifications after they are completely assembled. Traditionally, assembled hydraulic components are received at the test stand dry, i.e. (not pre-filled with hydraulic fluid) from the manufacturing process. They are then filled with hydraulic fluid, heated to an operating temperature (if that is part of the specification), are pressurized by hydraulic fluid at a specified testing pressure, and the minute leakage rates of hydraulic fluid is then measured.
The leakage flow rate is typically a tiny fraction of the components"" rated flow rate capacity. For example, a valve that provides a maximum fluid flow in operation of a gallon or more per minute may have a maximum permitted flow rate of only a few cubic centimeters of fluid per minute. In addition, the internal volume of the devicexe2x80x94the volume that must be filled with hydraulic fluid to purge all airxe2x80x94may be substantial as well.
At the same time, the leakage fluid flow rate measuring devices typically have a very small flow rate. A device intended to measure a maximum leakage flow rate of ten cubic centimeters per minute may have a maximum flow rate of perhaps twenty cubic centimeters per minute. This is primarily due to the small size, compact construction, and fragile nature of these precision measurement devices.
In a typical prior art test stand, a source of hydraulic fluid pressure is provided that is connected to the leakage flow rate measuring device, which is in turn connected to one of the ports of the hydraulic component that is to be tested. The hydraulic fluid source forces fluid through the measuring device and into the dry, just-assembled hydraulic component.
During the initial phase of this process, the quantity of fluid forced through the measuring device into the hydraulic component is quite high as the air inside the empty hydraulic component being tested is forced out. Once all of the air is forced out and the hydraulic component being tested is filled with hydraulic fluid, the actual leakage rate can be measured. This initial filling process often generates extremely high flow rates. Since the components are typically dry, there is no fluidic resistance to the initial inrush of fluid as the air is forced out. Air can be expelled through the air-filled gaps between the internal structures of a hydraulic component at an extremely high rate when pushed by the high pressure (typically around 1000 psi) of the hydraulic source.
There are significant problems in these prior art systems. First, since the measuring devices can only accommodate a tiny flow rate of hydraulic fluid, the maximum rate at which the hydraulic component can be filled during the initial phase is small. For a simple single spool bi-directional hydraulic control valve with pressure relief inserts and several check valves, this initial purging process can take as much as thirty or forty seconds. Again, this is because the flow rate through the measuring device must be severely limited to prevent damage to the device, or is inherently limited due to flow restrictions built into the measuring device. Since there is virtually no internal resistance to hydraulic fluid flow as the air is expelled from the hydraulic component being tested, however, the traditional test stands can produce very high fluid flow rates that can damage the leakage flow rate measuring device unless the flow through the measuring device is restricted. Restricting the flow through the measuring device, however, will unduly lengthen the filling time of the hydraulic component under test
Once the hydraulic component is filled, however, the leakage flow rate can be virtually instantaneously measured. Typically, only 1-3 seconds are needed for the leakage flow rate to stabilize and for the operator to take an accurate measurement of that flow rate. Thus, perhaps 90% of the time required to check the leakage flow rate of the hydraulic component being tested is due to the lengthy period required to fill the hydraulic component and purge it of all air. One way to avoid this problem is to replace the low-capacity measuring device with a high-capacity measuring device and to provide virtually unlimited flow into the hydraulic component during the initial fill process. In this manner, the leakage flow rate measuring device will accommodate the very high filling flow rate during the period in which the hydraulic component is being filled.
As might be expected, however, measuring devices able to accommodate much higher flow rates without being damaged typically have much lower resolution and therefore reduced accuracy of measurement. For example, a flow rate measuring device that can accommodate a high flow rate of 1 gallon per minute during the initially filling process will typically provide a corresponding flow rate measurement resolution of 10 cc per minute. For most components, this resolution is too large to accurately measure a leakage flow rate once the device is filled.
What is needed, therefore, is a test stand for testing hydraulic leakage flow rates of hydraulic components that combines the accuracy of a low flow rate hydraulic flow measuring device with a high flow rate initial fill and purging system. It is an object of this invention to provide such a test stand.
In accordance with the first embodiment of the invention, a hydraulic leakage rate testing system for testing and to test the leakage flow of hydraulic components is provided that includes a source of hydraulic fluid, a hydraulic coupler communicating with the source, a measuring circuit with a flow rate measuring device in communication with both the source and the coupler and a by-pass circuit that is in communication with both the source and the coupler. The measuring circuit and by-pass circuit are preferably connected in parallel to provide parallel flow paths between the source and the coupler. The measuring circuit preferably includes a valve that blocks fluid flow between the source and the coupler through the measuring device when the valve is closed and permits flow between the source and the coupler through the device when the valve is open. The measuring circuit also preferably includes an orifice disposed to restrict the flow through the measuring device. The measuring circuit may also include a pressure relief valve located to limit the maximum hydraulic pressure applied by this source to the measuring device. The pressure relief valve is preferably located between the measuring device and the source in the measuring circuit. Another valve may be provided to block hydraulic fluid flow provided by the source from passing through both the measuring circuit and the by-pass circuit. An electronic controller may be coupled to the valve in the measuring circuit and the valve in the by-pass circuit to block or permit the flow alternatively through either circuit in accordance with the stored digital program in the controller that opens and closes the valves in the by-pass circuit. The electronic controller may also be coupled to the measuring device to receive an electrical signal from the measuring device indicative of a flow rate through that device. The electronic controller may be configured to open the valve in the by-pass circuit long enough to fill the hydraulic component through the by-pass circuit. The electronic controller may be configured to close the valve in the by-pass circuit after the component is filled.